1. Field of the Invention
The present invention relates to passive optical network systems and sleep time determination methods, and more specifically, to a passive optical network system in which a sleep operation is performed while taking into account the link speed and the queue buffer capacity of each of an optical line terminal and an optical network unit, and a sleep time determination method used in the passive optical network system.
2. Description of the Related Art
While the speed and bandwidth of communication networks have been advancing, optical networks have been introduced in order to handle the increased speed and bandwidth. In the optical networks, one optical line terminal (hereafter called OLT) and one optical network unit (hereafter called ONU) perform point-to-point communications through an optical fiber. A passive optical network (hereafter called PON) is a star point-to-multipoint network formed of one OLT and a plurality of ONUs connected with optical fibers and an optical splitter, which branches optical fibers. Typical PONs include an Ethernet (registered trademark) PON (EPON) standardized in IEEE 802.3 and a gigabit capable PON (GPON) standardized in ITU-T G.984. In a PON, upstream frames sent from ONUs to the OLT and downstream frames sent from the OLT to the ONUs are multiplexed by wavelength division multiplexing (hereafter called WDM). In the downstream communication, the OLT sends identical data to all the ONUs connected through optical fibers. When each ONU receives the data, the ONU refers to destination information included in the preamble to discard frames other than those sent to the ONU and transfers only the frames sent to the ONU to the user. In the upstream communication, frames are multiplexed by time division multiplexing (hereafter called TDM), in which each ONU sends data at the specified time according to the transfer permission sent from the OLT. PON systems have been introduced, including those handling signals at a low speed such as 64 kbps, broadband PON (BPON) systems that send and receive fixed-length ATM cells at about 600 Mbps at a maximum, EPON systems that send and receive variable-length Ethernet packets at about 1 Gbps at a maximum, and GPON systems that can handle signals at a faster speed of about 2.4 Gbps. A high-speed PON that can handle signals at a rate ranging from 10 Gbps to 40 Gbps will be demanded in the future.
As the communication speeds of such PONs have increased, the power consumption of relay apparatuses disposed on transfer lines tends to increase. Since ONUs are installed in subscriber premises, a number of ONUs are disposed in the network. The ONUs require an available bandwidth for shorter periods than the OLT and an upper switch group. Therefore, the ONUs are left unused when they do not perform communication while using power.
Japanese Unexamined Patent Application Publication No. 2008-113193 discloses a method of reducing power consumption by setting the functional blocks of each ONU to a low power consumption mode when terminal equipment (hereafter called TE) is physically not connected to the ONU through a LAN cable, for example. Japanese Unexamined Patent Application Publication No. 2009-260970 discloses a method of setting an ONU to a sleep mode through a procedure in which the ONU sends a sleep request to the OLT and the OLT permits the request, for example.